This invention relates to dipole antennas and more particularly to coaxial dipole antennas.
The simple dipole antenna using open wire for the dipole elements has long been used by radio amateurs, shortwave listeners, and others. It is easy to construct, inexpensive, and sufficiently compact at the more popular frequencies (e.g., 33 ft. long at 14 MHz) to be erected or strung in a modest-size space. Its radiation pattern is bi-directional broadside to the length of the antenna. It has, however, rather narrow band width, i.e., its efficiency drops off rapidly at frequencies lower and higher than the design frequency. More recently, coaxial dipole antennas have come into rather wide usage. Such antennas have both the dipole elements and the feedline formed from the same type of coaxial cable. The dipole coaxial line, approximately 1/2 wavelength at the design frequency, has the outer jacket and the outer shield conductor cut and spread apart for a short distance each side of the center to form a gap thereby defining the two dipole elements. The inner and outer conductors of the coaxial feedline are respectively connected, as by soldering, to the outer shield conductors of the dipole elements on each side of the gap, these connections serving as feedpoint connections between the dipole elements of the coaxial feedline. At points spaced outwardly on the dipole elements from the feedpoint connections a small portion of the outer jacket, the outer shield conductor and the dielectric material surrounding the inner conductor are removed and the inner conductor and the outer shield conductors are electrically connected together, as by soldering, to form electrical junctions. Such coaxial dipole antennas have a number of advantages over the simple or open wire dipole antenna. For example, they are efficient over a much greater band width; have a positive gain (1.5 db) over a simple dipole operating under the same relative conditions; attenuate harmonics; decrease static charge build-up; and, when mounted in an inverted V configuration, are essentially nondirectional. However, removal of the jacket and outer shield to form the center gap of the dipole elements and the removal of substantial portions of the jacket, outer shield conductors and inner dielectric at the junctions considerably weaken the antenna at these points and make it susceptible to tensile failure. Also, the feedline connections and the electrical junctions are subject to rain and moisture damage at these points.